1. Field of the Invention
This invention relates to the use of digital Laminography for 3D imaging and inspection of baggage or objects, the technique of Laminography is also described alternatively as Tomosynthesis or Computed Tomography from limited projections. This invention is suited for automated systems used for the detection of explosives in baggage, and for 3D imaging for medical and non-medical applications.
2. Description of the Related Art
Most of the baggage screeners in use today generate a shadow image by interposing a baggage between a x-ray source and a linear array of radiation detectors, and translating or scanning the baggage over the said linear detector. This method generates a 2 D image from which it is difficult to detect for explosive and more so if the explosives are disposed off in the form of thin sheets, this problem is well known to the people skilled in the field.
To overcome the above drawback, Computed Tomography (CT) technique has been employed which can generate cross sectional images of slices through the object. Then from several such slice images, a 3D image can be built up from which the presence of explosives within the baggage can be inferred. However, since CT is a very time consuming technique, prescanning is employed to select only certain areas of baggage for scanning, for example as explained in U.S. Pat. No. 5,182,764 (Peschmann, et al.); U.S. Pat. No. 5,367,552 (Peschmann). The CT scanners described in these patents take 0.6 to 2 secs per rotation of the gantry required to generate one cross sectional or slice image through the object. If a bag is assumed to be 70 cm long, then for a desired throughput rate of 300 bags per hour, then there is only enough time to generate images of only 6-7 slices through the bag, this is obviously highly inadequate for a reliable inspection of the bag. To speed up the scan time and to enhance the performance, multi row detectors have been used as described in U.S. Pat. No. 5,818,897 (Gordon); U.S. Pat. No. 5,796,802 (Gordon) and U.S. Pat. No. 6,925,141 B2 (Bruder, et al.). However, all the CT based systems have major drawbacks in that the throughput is slow, they do not scan the entire baggage, only select areas are scanned to reduce the scan time, are very bulky, are very complex systems with rotating gantry and are very costly with each system costing around a million dollars or more. Further, the human eye is not accustomed to seeing the contents of bags in vertical slices or layers, it would be far better if horizontal layers were displayed in case visual inspection was required. However to generate horizontal layers, the CT of the entire bag would be required with the distance between slices kept very small requiring hundreds of slices to be taken which would take a very long time.
Another problem with the CT based techniques is that the systems are very bulky and complex, and it would be almost unthinkable to build one such system to generate a 3D image of large shipping containers or trucks. CT requires a rotating gantry around the object, to build such a massive gantry that rotates around a large shipping container at high speed and at a uniform rate without introducing too much vibrations that would otherwise corrupt the data, is a task so daunting that no one has ever built such a system. There are at present no systems available to build a 3D image of large shipping containers or trucks or cars.
A different approach from the CT methods of above has been described in the U.S. Pat. No. 6,088,423 (Krug, et al.) wherein 3 x-ray tubes and 3 sets of “L” shaped linear detectors are used for baggage screening, this method has been described as the Multi view Tomography. However, as is well known to one skilled in the art of CT, just 3 projections obtained are not enough to build a detailed 3D image of the object, and this technique therefore leaves to question the full efficacy of the technique. Further, the method using three x-ray tubes, is also very costly, bulky and has therefore found only limited use a airports and not suited for wide use as the ordinary x-ray scanners.
The above methods of CT, and the multiview Tomography do not generate high detail 3D images at high speeds. These methods have failed to take advantage of the fact that most bags are rather flat. Also these methods have not focused on generating slice images of horizontal layers within a bag which a human eye would find easier to decipher.
The methods of Laminography, more recently termed as Tomosynthesis, are good for generating slice images that are horizontal and can be suitable for imaging of the bags. The traditional methods of Laminography employed films and that made it almost impossible to use them for baggage screening. In recent times, Laminography has been used for inspection and imaging, but on a limited scale.
A high speed Laminography or tomosynthesis method using at least two linear detector arrays and at least one x-ray source, though the preferred embodiment uses four linear array detectors and two x-ray sources has been described in U.S. Pat. No. 5,583,904 (Adams). This method has been shown good for the inspection of thin objects like Printed Circuit Boards used in the electronic industry. This patent does not address the issue of explosive detection within a baggage.
A system very similar to the above, using one x-ray source and a stack of two or more linear detectors has been described in U.S. Pat. No. 6,940,942 B2. The system presented therein has been described for use in medical imaging and does not address the issue of explosive detection in baggage.